1. Field of the Disclosure
The present disclosure relates to a composite structure, and to a method of fabricating a composite structure.
2. Description of the Related Art
The present disclosure provides particular benefits in the field of composite aerofoils, and more particularly the field of composite fan blades for gas turbine engines. However, it is to be noted that in its broadest sense the disclosure is not limited to aerofoils or gas turbine fan blades, and can be used in connection with other types of composite structures also.
The use of composite materials for aerofoils in the form of fan blades for gas turbine engines, particularly those used for aircraft propulsion, is becoming increasingly attractive for engine manufacturers. Carbon-fibre-reinforced-plastic (CFRP) is of particular interest to engine designers in this regard. Advantages attributable to composite fan blades over more traditional titanium fan blades include lighter weight, improved efficiency, improved maintenance intervals, a reduction in noise and an improved performance characteristic in the event of ingestion of a foreign object such as a bird.
However, gas turbine fan blades are subject to an extremely small acceptable tolerance for dimensional variation, and it has been found that this can make it difficult and/or time-consuming and expensive to fabricate a composite fan blade which will be certain of satisfying the acceptable design tolerance on its thickness. It is sometimes the case that a composite fan blade must be scrapped if it cannot be made to satisfy the dimensional requirement of the blade design.
In order to ensure dimensional compliance with a fan blade design, it is common to machine composite preforms by removing material from their surfaces until the preform has an acceptable thickness. It has been found that the thickness of typical carbon-fibre plies used in the manufacture of CFRP fan blades can vary by approximately 4%, and this variation in ply thickness can affect the thickness of the manufactured component. To ensure compliance with design tolerances it is therefore known to machine the outer surfaces of the product after is has been moulded, in order to remove material and reduce its thickness to a size which falls within the design tolerance. This machining process can jeopardise the integrity of the component by reducing its strength through the removal of material, and can also induce undesirable stress in the product.